The invention relates to “active implantable medical devices” as defined by the Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities, and particularly implantable devices that continuously monitor the heart rate and if necessary deliver electrical stimulation, resynchronization and/or defibrillation pulses to the heart when a rhythm disorder is detected by the device.
The disclosure relates to devices that are autonomous capsules intended to be implanted in a heart chamber, especially a ventricle. These capsules are free of any mechanical connection to an implantable main device (e.g., a housing of the stimulation pulse generator) or non-implantable main device (e.g., an external device such as a programmer or monitoring device for patient remote monitoring). These devices are called “leadless capsules” to distinguish them from electrodes or sensors disposed at the distal end of a conventional probe (lead), which is traversed throughout its length by one or more conductors galvanically connecting the electrode or sensor to a generator connected to an opposite, proximal end of the lead. A detection/stimulation electrode in contact with the wall of the ventricle enables it to detect the presence or absence of a spontaneous depolarization wave of the cardiac cavity, as well as the time of occurrence of this wave (ventricular or atrial marker).
The electrode also allows the delivery of a stimulation pulse in the event of absent or late spontaneous depolarization to cause contraction of the cardiac cavity.
Note, however, that the autonomous nature of the capsule is not inherently a necessary feature of the present disclosure.
The management of stimulation energy is a critical aspect of any implantable pacemaker, because it has a direct impact on the power consumption of a battery, and thus on its overall lifespan.
Power consumption is particularly critical in the case of a leadless capsule where, unlike conventional pacemakers, the energy required for the issuance of stimulation is 70% of the total energy consumed. In addition, the very small dimensions of the leadless capsule impose restrictions on the size of the battery and thus the capacity, as the battery in the leadless capsule often occupies more than 70% of the volume of the device.
In fact, if it was possible to reduce, for example, half the energy required for stimulation, then the size of the battery could be reduced by about 40% while keeping the same longevity, which would reduce the volume of the capsule to about 0.6 cm3 (against 1 cm3 in the best case today), with all performances being equal.
To minimize the energy dedicated to stimulation as much as possible, while maintaining the effectiveness of delivered electrical pulses, a technique called “cycle to cycle capture” may be employed, which maintains the stimulation energy at a minimum level continuously checking, if the stimulation was effective (“capture”) or not, after each stimulation. If no depolarization wave has been induced by stimulation of the cardiac cavity (non-capture), the implant delivers, during the same cardiac cycle, a stimulation of a relatively high energy to ensure the triggering of a depolarization. Then, by successive iterations, the stimulation energy is gradually reduced in each cardiac cycle, to converge to an energy close to a limit or “triggering threshold” needed to cause depolarization of the cardiac cavity.
Various capture test techniques have been proposed. A signal provided by a sensor directly detecting the mechanical contraction of the myocardium can be used, which allows information about the response of the cardiac cavity to stimulation to be obtained immediately, disregarding the blanking periods and other limitations inherent to the collection of an electric signal. In other words, the purpose is to use a functional signal representative of cardiac mechanics, instead of a signal originating from the electrical propagation of a depolarization wave.
EP 2412401 A1 (Sorin CRM) describes such a device including ventricular capture test methods operating by analysis of an endocardial acceleration (EA) signal. The EA signal can be collected by an endocardial lead equipped with a distal pacing electrode implanted into the ventricle, incorporating a microaccelerometer for measuring endocardial acceleration.
The capture test is based on the analysis of the EA signal, including its successive components (EA components) corresponding to the main heart sounds that are possible to recognize in each cardiac cycle (S1 and S2 sounds of a phonocardiogram). Amplitude variations of a first component (EA1 component) are closely related to changes in pressure in the ventricle, while a second component (component EA2) occurs during the isovolumetric ventricular relaxation phase. The analysis can also take into account the secondary component (called EA4 or EA0 component) produced by a contraction of the atrium, as in the case of EP 2189182 A1 (Sorin CRM), which describes a device provided with analysis methods for recognizing the presence (or absence) of a EA4 component in the EA signal in order to deduce the presence (or absence) of a contraction of the atrium, subsequent to an application of an electrical pulse to the latter by the atrial pacing methods.
For the ventricular capture test described by EP 2412401 A1 cited above, the EA1 and EA2 components of the EA signal are analyzed to extract various relevant parameters such as the peak-to-peak amplitude PEA1 and PEA2 of the EA1 and EA2 components, the temporal interval between the PEA1 and PEA2 peaks, the half height width of the components EA1 and/or EA2, the instants of beginning and of end of these components, etc. The parameters extracted may also be morphological parameters representative of the waveform of the EA signal or of its envelope.
In the technique described by EP 2412401 A1 cited above, the different parameters are calculated and grouped as a representative vector, creating a point of a multidimensional vector space. The vector space is then analyzed by application of classification algorithms to determine a boundary in the vector space between capture and non-capture. For each cycle, a parameter vector is formed from the collected EA signal and the position of the parameter vector in the vector space is evaluated to determine whether capture is present or absent.
This type of ventricular capture test is very powerful. However, its implementation, by complex algorithms of signal analysis and classification in a multidimensional space, involves complex numerical calculations, resulting in high consumption of the implant processor, typically on the order of 2 μW (which is compared with the energy is consumed for the delivery of stimulation, on the order of 5 μW).
To significantly reduce the consumed energy, especially for a leadless capsule pacemaker, the problem is to find a method of verification of the cycle to cycle ventricular capture that not only reduces energy stimulation to a minimum, but also does not increase the consumption of the electronic circuit, in particular of the digital processor.
It is desirable that the consumption of the electronics associated with the cycle to cycle capture test function does not exceed a few hundreds of nanowatts. In fact, if the energy dedicated to stimulation can also be reduced at a value of 1 μW to 2 μW (depending on the level of the stimulation threshold), the size of the battery can be reduced 40-60% compared to that of current devices.
Other capture verification techniques by implementation of a detection of the mechanical contraction of the heart are described in particular in U.S. Pat. No. 5,549,652 (a cycle to cycle capture test, but without detailed description of a specific method) and U.S. Pat. No. 6,650,940 B1 (a periodic conventional capture test by gradual reduction of energy over several cycles).